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Research ArticleLaboratory Leaching Tests to Investigate Mobilisation andRecovery of Metals from Geothermal Reservoirs

Máté Osvald ,1 Andrew D. Kilpatrick,2 Christopher A. Rochelle,2 János Szanyi,1

Tamás Medgyes,1 and Balázs Kóbor1

1University of Szeged, Department of Mineralogy, Geochemistry and Petrology, Egyetem u. 2, Szeged H-6722, Hungary2British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK

Correspondence should be addressed to Máté Osvald; [email protected]

Received 27 July 2018; Accepted 24 September 2018; Published 13 December 2018

Guest Editor: Domenico Montanari

Copyright © 2018 Máté Osvald et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The H2020 project “Combined Heat, Power and Metal extraction” (CHPM2030) aims at developing a novel technology whichcombines geothermal energy utilisation with the extraction of metals in a single interlinked process. In order to improve theeconomics of geothermal-based energy production, the project investigates possible technologies for the exploitation of metal-bearing geological formations with geothermal potential at depths of 3–4 km or deeper. In this way, the coproduction of energyand metals would be possible and could be optimized according to market demands in the future. This technology could allowthe mining of deep ore bodies, particularly for critical metals, alongside power production, while minimizing environmentalimpact and costs. In this paper, we describe laboratory leaching experiments aimed at quantifying the relative rates andmagnitudes of metal release and seeing how these vary with different fluids. Specific size fractions (250–500 μm) of groundmineralised rock samples were investigated under various pressures and temperatures up to 250 bar and 250°C. Initialexperiments involved testing a variety of potential leaching fluids with various mineralised samples for a relatively long time(up to 720 h) in batch reactors in order to assess leaching effectiveness. Selected fluids were used in a flow-through reactor withshorter contact time (0.6 h). To ensure possible application in a real geothermal reservoir, a range of fluids were considered,from dilute mineral acid to relatively environmentally benign fluids, such as deionised water and acetic acid. The main findingsof the study include fast reaction time, meaning that steady-state fluid compositions were reached in the first few hours ofreaction and enhanced mobilisation of Ca, Cd, Mn, Pb, S, Si, and Zn. Some critical elements, such as Co, Sr, and W, were alsofound in notable concentrations during fluid-rock interactions. However, the amount of these useful elements released is muchless compared to the common elements found, which include Al, Ca, Fe, K, Mg, Mn, Na, Pb, S, Si, and Zn. Even thoughconcentrations of dissolved metals increased during the tests, some remained low, and this may present technical challengesfor metal extraction. Future efforts will work toward attaining actual fluids from depth to more tightly constrain the effect ofparameters such as salinity, which will also influence metal solubility.

1. Introduction

The strategic objective of the CHPM2030 project is todevelop a novel technological solution (combined heat,power, and metal extraction from ultradeep ore-bearingrocks), to make geothermal energy more attractive, and toreduce Europe’s dependence on the import of metals and fos-sil fuels [1].

The idea of using geothermal brines for mineral extrac-tion has existed for decades. One key element of interest islithium [2–4], but a wide spectrum of elements that may besuitable for extraction is present in geothermal reservoirsand fluids [5, 6].

Current demand for metals is driving an expansion inmining operations aided by scientific and technical advancesin, for example, the use of robotics, nano-mining, laser

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mining [7, 8], etc. These developing technologies reduce theexposure of miners to hazardous underground environmentsand make possible the selective transport of valuable metalsto the surface rather than moving large amounts of materialwhich will eventually go to waste.

In the envisioned CHPM technology, an enhanced orengineered geothermal system (EGS) is established within ametal-bearing geological formation at depths of 3–4 km ormore (Figure 1). Based on geological and hydrogeologicalsettings, EGS could be hydrothermal or petrothermal (a hotdry rock (HDR) system) [9]. The concept involves themanipulation of a theoretical petrothermal system such thatthe coproduction of energy and metals will be possible [10].Through experiments at the laboratory scale, we have inves-tigated the leaching potential of various fluids and whethersuch enhancement of geothermal systems can make themmore attractive economically, i.e., whether metals can be lea-ched from orebodies in economic concentrations over pro-longed periods, and if leaching might increase the system’sperformance over time (through, for example, silicate disso-lution and permeability enhancement), negating or reducingthe use of more common methods of reservoir stimulation.

A key aspect of the CHPM2030 concept is that metals canbe transported in solution from mineralised structures atdepth to surface infrastructure where they can be extracted(Figure 1). Based on current technology, which often relieson exchange or adsorption processes, the extraction processwill be more effective with higher dissolved concentrationsof metals [11] and hence with faster rates of dissolution ofmetal-bearing minerals. However, too large a dissolved load

may lead to problems of precipitation within productionboreholes or surface infrastructure, and hence, it increasesmaintenance needs and costs. Thus, there is a need to balancethe potential for increased revenue generation from recover-ing more metals against potential increased costs resultingfrom increased maintenance operations. There is also a needto consider the wider physical environment in which thesystems will need to operate as well as issues of public accep-tance [12]. This includes being mindful of environmentalconsiderations and the carefully controlled use of additivesthat are relatively environmentally benign.

Factors underpinning the above aspects are the rates andmagnitudes of metal release, and laboratory experimentssimulating in situ conditions are a useful way to providewell-constrained data to help understand these. Such experi-ments also allow us to test different fluid compositions inorder to ascertain if there are specific additives that mayimprove the metal recovery process [13].

Our approach has been to work initially at lower temper-atures with a focus on a few mineralised samples (mainly onmaterial from Cornwall, southwest England) in order toinvestigate the leaching potential of various fluids. Here, wepresent results from experiments using samples of minerali-sation from Cornwall reacted with deionised water, aceticacid, and a mixture of hydrochloric and nitric acid. Samplesfrom the Banatitic Magmatic and Metallogenetic Belt inRomania (BMMB Masca-Cacova Ierii) and Hungary (Ruda-bánya and Recsk) were also tested with deionised water andacetic acid. These fluids have been chosen to represent thescale from very benign (deionised water) to more aggressive

Cool geothermalfluid

Electric grid

Electric grid

AirAir

Condenser

GeneratorTurbineWorking fluid

Electrolyticmetal recovery

Metal recoveryby gas diffusion

Heat exchanger

Salt gradient

Injection well

Rock formation

Ultradeep orebody

Production well

Natural heat exchangerHeat flow

Hot geothermalfluid

Pump

Cooling tower

Water

Figure 1: Schematic representation of the CHPM concept. The information presented in this report relates to the release of metals from the“ultradeep orebody” and into the recirculating geothermal fluid.

2 Geofluids

but still reasonably acceptable (0.1M acetic acid or ‘vinegar’)and to very aggressive and environmentally unacceptablemineral acid.

Experiments were run using both batch equipment, wherematerials are reacted in a closed system, and fromwhich sam-ples are withdrawn regularly, and also flow-through equip-ment, where fluid is passed once through the system.

2. Materials

The solids used in the experimental work are detailed inTable 1. Samples generally consist of either massive minerali-sation or mineralised material together with some surround-ing country rock.

All samples were repeatedly crushed in a tempered steeljaw crusher to obtain a powdered fraction of <500μm. Thisfraction was then sieved to produce a 500–250μm fraction,which was used for the experimental and analytical work.This fraction was cleaned to remove fines and surfaceimpurities, by repeated rinsing in acetone, until the super-natant ran clear. These “washed” samples were then ovendried at 30°C.

Solid samples will be referred to by their unique three-digit identifier throughout this report. Samples collected byBritish Geological Survey were from sites in South WestEngland and labelled HTL315, HTL319, and HTLMix whichis a mixed sample from materials representative of a minera-lised quartz vein (containing galena, sphalerite, and somechalcopyrite) found at Herodsfoot, southwest England. Themixture was used to provide more representative “bulk”min-eralogy for use in experiments. HTL321 originates from askarn deposit in the BMMB Masca-Cacova Ierii in Romaniawhich is a magnetite deposit also enriched in sulphides withvisible chalcopyrite. HTL322 is from Rudabánya, Hungary,from a Mississippi Valley Type (MVT) deposit. The sampleis characterised by banded baritic lead ore from a metaso-matic deposit hosted by limestone; galena grains in darkbands can be recognized with coarse-grained white baritelenses and fine-grained limonitic matrix. HTL324 fromRecsk, Hungary, represents porphyry mineralisation

sampled from an intrusion related to porphyry copperdeposits and includes a breccia with sulphide matrix andveins [14]. Starting materials were characterised using X-ray diffraction for bulk mineralogy and BET (Brunauer–Emmett–Teller) analysis for surface area.

Details of the solid samples, including their samplinglocation, geological setting, and a summary of their bulkcomposition, as determined by XRD, can be found inTable 1. All samples were collected from the surface, gener-ally from mine dumps or rockfalls adjacent to exposures.Efforts were made to ensure that the material used for theexperiments was as pristine as possible, i.e., materials at ornear (within ~10 cm) weathered surfaces were avoided. Avariety of solutions were used in the experiments in orderto test their relative potential for liberation of metals fromore-bearing deposits. Most of these were created using oneor two reagents dissolved or diluted to the desired concentra-tion. The fluids used in the experiments reported here as wellas the temperature/pressure conditions of the experimentsusing various solids are summarised in Table 2.

3. Method

3.1. Batch Experiments. Two different methods were used forthe batch experiments, which are chosen according to theexperimental conditions (i.e., pressure and temperature)required. Initial experiments were carried out at atmosphericpressure, using high-density polyethylene (HDPE) bottlesfixed into a rotating mixing assembly. Solid samples werecarefully weighed and added to the appropriate fluid in a40 : 1 fluid : rock ratio. The relatively high fluid to rock ratiowas chosen to meet fluid sampling requirements while mini-mising changes in reaction rate due to relative changes influid : rock ratio due to sampling. The experimental “charge”in these experiments consisted of an accurately knownamount of granulated rock sample (around 5 g) togetherwith 200ml of reactant solution. The tops of the HDPE bot-tles were securely tightened, the vessels were arranged sym-metrically on the mixer, and the entire assembly was placedinto a thermostatically-controlled fan-assisted oven. When

Table 1: Major geological and mineral properties of the samples.

Sample ID Sample locality Geological settingSummary of bulk mineralogy(as determined by XRD)

HTLMixHerodsfoot,SW England

Baked sediments with partialquartz vein

87% quartz, 5% muscovite, 2% dolomite, 5% galena,minor albite, chlorite, pyrite, and sphalerite

HTL315South Caradon,SW England

Mainstage mineralisationassociated with granite bodies

70% quartz, 7% schorl, 5% chlorite, 2% calcite,10% pyrite, 5% arseonpyrite, and minor greigite and biotite

HTL319Cligga Head,SW England

Tin-tungsten mineralisationassociated with granite bodies

88% quartz, 2% muscovite, 3% cassiterite,3% columbite, and 4% ferberite

HTL321Masca-Cocovaleni,

RomaniaMineralised skarn country rock

22% dolomite, 49% pyrite, 27% magnetite,minor quartz, calcite, and barite

HTL322Rudabánya,NE Hungary

Carbonate hosted lead-zincmineralisation

8% quartz, 2% calcite, 68% magnesite, 6% cerussite,1% sphalerite, 1% columbite, 11% barite,

2% magnetite, and minor dolomite

HTL324 Recsk, NE Hungary Porphyry sulphide polymetallic ore74% quartz, 5% calcite, 9% pyrite, 11% magnetite,

minor albite, dolomite, and sphalerite

3Geofluids

running, the mixer turned at approximately six revolutionsper minute—enough to ensure good mixing between solidand solution without causing too much mechanical damageto the solid grains.

Higher temperature and pressure experiments utilisedtitanium batch reactors inside thermostatically-controlledfan-assisted ovens [15, 16]. The basic layout of the batchreactors used is shown schematically in Figure 2. Viton O-rings are used between the vessel body and vessel head to pre-vent loss of pressure. A large retaining ring is screwed ontothe top of the vessel to keep the vessel body and vessel headtogether when pressurised. This equipment was used forthe experiments at 100°C, 150°C, and 200°C.

Loading the vessel consisted of adding accurately knownamounts of granulated rock (approximately 8.75 g) and syn-thetic groundwater or other leaching solution (350ml) plus amagnetic stirrer bead in the experiments carried out below200°C. The head of the reaction vessel was then pushed on,and the retaining ring securely screwed down. The headspaceof the vessel was flushed with nitrogen prior to pressurisa-tion. A titanium dip tube (and associated valve), fitted witha PTFE filter assembly, was added to each vessel.

To minimise mechanical damage to the solid, the stirrerbead was both held in a small cage and only activated forapproximately 2 minutes in every 4 hours. For the experi-ments conducted at 200°C, the stirrer assembly and stirrerbead, as well as the filter assembly, had to be removed, andinstead, the vessels were periodically agitated by hand (onaverage about once per day). Nitrogen was used to pressurisethese batch experiments with experimental pressure beingcontrolled via an ISCO 360D syringe pump. The N2 usedwas classified as “oxygen free” (99.998% pure).

At the end of each experiment, as much of the solution aspossible was removed prior to cooling and depressurisationof the vessel. Once well below 100°C (i.e., the boiling pointof the leachate being used), the vessel was slowly depres-surised, dismantled, and reacted rock grains recovered forsubsequent analysis. Batch experiments were run for around600–1000 hours.

3.2. Flow-through Experiments. Leaching processes were alsoinvestigated under continuous flow conditions using a flow-through reactor (Figure 3). The reaction took place in a stain-less steel high-pressure liquid chromatography (HPLC)

column 250mm in length and with an inner diameter of21.2mm. The pressure in the column was maintained by anEcom Kappa 10 Single-Plunger HPLC pump. A 50 cm stain-less steel capillary and a fluid back-pressure regulator werefitted at the outflow of the column. The length of this tubingwas used to allow outflowing fluid to cool to below 90°Cbefore being depressurised. Heating bands were attached tothe column and controlled by a thermostat (WH-1435DPID digital thermostat with ±1°C control regulation). Thishigh-pressure high-temperature device was loaded withapproximately 150 g of the rock sample and operated at atemperature of approximately 250°C and a pressure of250 bar. These parameters correspond to depths of around2.5–3 km in an average geothermal field [9, 17]. During theexperiments, the flow rate in the reactor was 0.5ml per min-ute, which resulted in a contact time between the fluid androck of 30–50minutes, allowing the collection of sufficientsample volumes for chemical analyses.

3.3. Sampling and Analysis. For sampling, the experimentscarried out using the rotating shaker setup; rotation wasstopped, and the bottles were removed from the assemblyone at a time to minimise any cooling following removalfrom the oven. Upon removal, each bottle was unsealed,and a sample was removed using a polyethylene syringeand subsequently filtered using a 0.2μm nylon syringe filterprior to subsampling for analyses.

For experiments carried out using titanium vessels, avalve on top of the vessel (attached to an internal titaniumsampling tube) was opened to a syringe attached to the valvevia a length of polyetheretherketone (PEEK) tubing. Anaccurately known quantity (typically 1–5ml) of fluid wasallowed to flow into the syringe in order to flush the sampletube, valve, and tubing. This syringe was removed and dis-carded. A second syringe was then attached and used to with-draw an accurately known amount (approximately 10ml) offluid. This sample was subsequently filtered using a 0.2μmnylon syringe filter.

Once a sample of filtered fluid was obtained, each wassplit into several sub-samples for ion chromatography (IC),inductively coupled mass spectrometry (ICP-MS), alkalinity(carried out by titration against sulphuric acid), and reducediron (carried out colorimetrically using ultraviolet spectrom-etry) analyses as well as analysis of pH and Eh. Subsamples

Table 2: Summary of experimental materials and conditions used.

Solvent Deionised water 0.1M acetic acid0.01M HCl,

0.003M HNO30.1M HCl, 0.03M

HNO3

SampleID

70°C,1 barbatch

100°C,200barbatch

200°C, 250 barflow-through

70°C,1 barbatch

150°C,200 barbatch

250 °C, 250 barflow-through

100°C, 200 barbatch

100°C,200 barbatch

200 °C,200 barbatch

HTLMix ✓ ✓ ✓ ✓ ✓ ✓ ✓

HTL315 ✓ ✓ ✓ ✓

HTL319 ✓ ✓ ✓

HTL321 ✓ ✓ ✓

HTL322 ✓ ✓

HTL324 ✓ ✓ ✓

4 Geofluids

Magnetic stirrer

Gas

Disaggregatedcaprock sample

Sampletube

Filter

Gas inletLiquidsampleoutlet

Leachant

Stirrerbead

Figure 2: Schematic diagram and photograph of a titanium batch reactor.

(a) (b)

Figure 3: Flow-through reactor (a) and temperature control on top of the HPLC pump used (b).

5Geofluids

Table3:Physicalp

ropertiesof

each

fluid-rock

interactionbatchreaction

s.

LeachateID

Rock

sampleID

Rock

sampleorigin

Rocksampletype

Rocksample

characteristics

Solvent

Pressure

(bar)

Tem

perature

(°C)

Residence

time(h)

HTLM

ix+DIwater

HTLM

ixUKHerod

sfoot

Baked

sedimentswith

mineralised

veining

Mixture

ofvariou

ssamples

containing

galena,sph

alerite,

andsomechalcopyrite

Deion

ised

water

170

670

HTLM

ix+0.1M

aceticacid

0.1M

aceticacid

720

HTLM

ix+0.1M

aceticacid

200

150

1000

HTLM

ix+0.013M

mineral

acid

0.01

MHCl+

0.003M

HNO3

100

770

HTLM

ix+0.13

Mmineral

acid

0.1M

HCl+

0.03

MHNO3

770

HTLM

ix+0.13

Mmineral

acid

0.1M

HCl+

0.03

MHNO3

200

530

HTL3

15+DIwater

HTL3

15UKSouth

Caradon

Granite-hosted

mineralisation

Mainstage

mineralisation

associated

withgranitebodies

Deion

ised

water

170

670

HTL3

15+0.1M

aceticacid

0.1M

aceticacid

720

HTL3

19+DIwater

HTL3

19UKCligga

Head

Granite-hosted

mineralisation

Tin-tun

gstenmineralisation

associated

withgranitebodies

Deion

ised

water

170

670

HTL3

19+0.1M

aceticacid

0.1M

aceticacid

720

HTL3

21+0.013M

mineral

acid

HTL3

21RO

CacovaIerii

Skarn

Magnetitedepo

sitenrichedin

sulphideswithvisible

chalcopyrite

0.01

MHCl+

0.003M

HNO3

200

100

770

HTL3

24+0.013M

mineral

acid

HTL3

24HURecsk

Porph

yry

Intrusion-relatedpo

rphyry

copp

erdepo

sit

0.01

MHCl+

0.003M

HNO3

200

100

650

6 Geofluids

for ICP-MS analysis were diluted using deionised water andacidified using HNO3. Subsamples for alkalinity and ionchromatography (IC) analyses were diluted using deionisedwater. Subsamples for analysis of reduced iron werediluted and prepared for analysis using deionised waterand 2,2-bipyridyl solution.

At the end of each experiment, as much of the fluidphase was removed as possible to minimise the formationof unwanted precipitates during cooling and depressurisa-tion. The vessels were then cooled as rapidly as possibleto below 80°C and then depressurised. After the openingof the reaction vessels, any residual fluid was sampled(and then subsampled and preserved as per samplesdescribed above) to allow characterisation of any chemicalchanges in the system during depressurisation and cooling.The reacted solids were removed from the vessel, a sub-sample of which was rinsed using acetone, and then ovendried at 30°C.

For quantitative whole-rock X-ray diffraction (XRD)analysis, >5 g samples of the starting solids were ball-milledand then micronized underwater to a fine powder (<10μm).XRD analysis was carried out using a PANalytical X’PertPro series diffractometer equipped with a cobalt-target tubeand operated at 45 kV and 40mA. Derivation of quantitativemineralogical data was accomplished by using the leastsquares fitting process applying the Rietveld refinement tech-nique [18]. A subsample of the crushed starting solids wasalso dissolved using hydrofluoric acid digestion, and theresulting liquid was analysed using ICP-MS as per the fluidsamples from the experiments.

4. Results

4.1. Batch Experiments. Batch experiments were conductedusing deionised water, acetic acid (in 0.1M concentration),and mineral acid (a mixture of 0.13M HCl and 0.013MHNO3) on the range of samples HTLMix, HTL315,HTL319, HTL321, and HTL 324 at 70°C, 100°C, 150°C, and

200°C under 1 bar (70°C experiments only) and 200 bar pres-sure (Table 3). In this study, the elements appearing in thehighest concentration were Al, B, Ba, Ca, Cd, Cr, Cu, Fe, K,Mg, Mn, Na, Ni, Pb, Rb, S, Si, and Zn. The economic valueof these elements is debatable, due to limited utility or wideavailability, and these elements are here referred to as “com-mon” elements. Elements with higher value but appearing inlower concentrations, such as Ag, Co, Ga, Mo, Sb, Sr, V, andW elements, were selected as desirable and referred to as “atrisk” elements in this paper based on the evaluation by Euro-pean Commission et al. [1]. Data about the composition ofsamples from batch reactions, on which the following figuresare based, can be found in Tables 4 and 5. To illustrate resultsfrom analyses, spider plots were used, where individual ele-ments are arranged in a ring around a central point repre-senting zero concentration of all the elements. Thus,concentrations increase away from the centre of the plot (inthis study on a logarithmic scale).

4.1.1. Leaching Tests Using Deionised Water as Solvent.Leaching experiments were carried out using deionised wateron UK samples HTL315, HTL319, and HTLMix at 70°C. Thetotal concentration of “common” elements found in theleachate shown in Figure 4 corresponds to approximately70 ppm in the case of HTL315, 5.4 ppm in the case ofHTL319, and 29 ppm in the case of HTLMix. In HTL315,Fe and Si were the elements detected in higher concentration,accounting for 58% and 24% of the total “common” elemen-tal concentration, respectively. In HTL319, Si was the mostabundant element, accounting for 96% of the total “com-mon” elemental concentration. For the leachate producedby reaction with HTLMix, Mg was the most abundant ele-ment, with K and Si were also detected, accounting for67%, 16%, and 15% of the total “common” elemental concen-trations, respectively.

Reaction at 70°C temperature under 1 bar pressure withHTL315, HTL319, and HTLMix resulted in the mobilisationof approximately 1070 ppb, 180 ppb, and 170 ppb of the

Table 4: Concentration of the “at risk” elements in each leachate from the batch reaction.

Element Ag Co Ga Mo Sb Sr V W Total “at risk”Sample PPB PPB PPB PPB PPB PPB PPB PPB PPB

HTLMix +DI water ∗ ∗ ∗ ∗ 116.41 51.30 ∗ ∗ 167.71

HTLMix + 0.1M acetic acid ∗ 77.29 17.15 ∗ 20.41 92.04 ∗ 67.67 274.56

HTLMix + 0.1M acetic acid 200°C ∗ 26.48 4.00 1.04 656.28 141.60 ∗ ∗ 829.40

HTLMix + 0.013M mineral acid ∗ 3.76 ∗ 2.64 32.10 84.80 ∗ 0.24 123.54

HTLMix + 0.13M mineral acid 100°C ∗ 159.07 12.00 17.48 3.49 149.20 ∗ 0.28 341.52

HTLMix + 0.13M mineral acid 200°C ∗ 979.95 8.00 176.00 7100.00 220.00 ∗ 7.98 8491.92

HTL315 +DI water ∗ 1001.28 ∗ ∗ 71.93 ∗ ∗ ∗ 1073.21

HTL315 + 0.1M acetic acid ∗ 1069.75 ∗ ∗ 90.06 ∗ ∗ 91.59 1251.40

HTL319 +DI water ∗ ∗ ∗ ∗ 2.41 ∗ ∗ 181.83 184.24

HTL319 + 0.1M acetic acid ∗ ∗ ∗ ∗ ∗ ∗ ∗ 128.04 128.04

HTL321 + 0.013M mineral acid ∗ 11.64 ∗ 2.60 43.51 101.20 ∗ ∗ 158.95

HTL324 + 0.013M mineral acid ∗ 5.52 ∗ 0.44 0.16 58.40 ∗ ∗ 64.52∗: concentration was under the detection limit.

7Geofluids

Table5:Con

centration

ofthe“com

mon

”elem

entsin

each

leachatefrom

thebatchreaction

.

Element

Al

BBa

Ca

Cd

Cr

Cu

FeK

Mg

Mn

Na

Ni

Pb

Rb

SSi

Zn

Sample

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

HTLM

ix+DIwater

105

∗46

∗∗

∗7

∗4684

19,343

∗∗

∗418

12∗

4309

∗

HTLM

ix+0.1M

aceticacid

1642

∗∗

91,652

17∗

15∗

3401

51,998

8309

∗117

871,112

22∗

14,277

3131

HTLM

ix+0.1M

aceticacid

200°C

460

∗2162

112,006

22∗

1213,059

5960

49,950

6996

12,000

7118,499

15∗

75,692

6261

HTLM

ix+0.013M

mineralacid

∗∗

970

80,004

22

∗120

∗32,273

5679

4000

880

∗4001

∗120

HTLM

ix+0.13

Mmineralacid

100°C

76,984

∗958

124,006

1553

1094

34,406

12,103

15,601

112,961

8458

6400

1113

3,676,056

824001

81,044

323,065

HTLM

ix+0.13

Mmineralacid

200°C

29,722

∗1412

∗809

304

924

848,648

76,003

62,787

25,897

∗42,850

805,280

524

232,036

210,993,801

183,156

HTL3

15+DIwater

5287

∗∗

∗∗

∗3996

40,705

∗∗

701

∗27

∗4

∗17,198

1922

HTL3

15+0.1M

aceticacid

7412

∗∗

∗∗

∗10,327

56,082

∗∗

779

∗31

∗5

∗21,004

1115

HTL3

19+DIwater

∗∗

∗∗

∗∗

11∗

∗∗

206

∗∗

∗8

∗5181

∗

HTL3

19+0.1M

aceticacid

3079

∗∗

∗∗

36132

4767

∗∗

837

∗∗

∗31

∗8822

∗

HTL3

21+0.013M

mineralacid

∗∗

866

92,005

232

∗8

1200

37,648

6726

3600

64

∗4001

∗760

HTL3

24+0.013M

mineralacid

26,623

∗1294

32,002

712

13143,755

2400

22,871

1311

11,600

314

64001

∗1992

∗:con

centration

was

underthedetectionlim

it.

8 Geofluids

selected “at risk” elements, respectively. Of these elements,Co, W, and Sb were detected at the highest concentrations(Figure 5).

4.1.2. Leaching Tests Using Acetic Acid as Solvent. The totalconcentration of “common” elements in the final sample

taken at 70°C, shown in Figure 6, corresponds to approxi-mately 100 ppm in the case of HTL315, 18 ppm in the caseof HTL319, and 1050ppm in the case of HTLMix. InHTL315, Fe and Si were the elements with the highest con-centration, yielding 58% and 22% of the total “common” ele-ments. In HTL319, Si was the most dominant among the

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn

HTL315 + DI water

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn

HTL319 + DI water

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn

HTLMix + DI water

0.00.1

10.01000.0

0.00.1

10.01,000.0

0.00.1

10.01000.0

Figure 4: Common elemental composition (in ppm) of each leachate reacted with deionised water at 70°C temperature under 1 bar pressurein batch rotating shakers after 670 hours (absence of data points reflects concentrations below the limits of detection).

Ag

Co

Ga

Mo

Sb

Sr

V

W

Ag

Co

Ga

Mo

Sb

Sr

V

W

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL315 + DI water HTL319 + DI wate HTLMix + DI water

110

100

10000

1000

110

100

10000

1000

110

100

10000

1000

Figure 5: Concentration (in ppb) of the “at risk” elements in each sample reacted with deionised water at 70°C temperature under 1 barpressure in batch rotating shakers after 670 hours (absence of data points reflects concentrations below the limits of detection).

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn

HTL315 + 0.1 M acetic acid

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn

HTLMix + 0.1 M acetic acid

0.00.1

10.01000.0

0.00.1

10.01000.0

0.00.1

10.01000.0

Figure 6: Elemental composition (in ppm) of each leachate reacted with 0.1M acetic acid at 70°C temperature under 1 bar pressure in batchrotating shakers after 720 hours (absence of data points reflects concentrations below the limits of detection).

9Geofluids

“common” elements, accounting for 50% of the total dis-solved concentration. In the case of HTLMix, large amountsof lead were leached, with concentrations of 870 ppm Pb inthe final sample, accounting for 83% of the total concentra-tion of “common” elements observed.

Reaction with 0.1M acetic acid at 70°C temperatureunder 1 bar pressure resulted in leached concentrations of

approximately 1250 ppb of the selected “at risk” elementsin the case of HTL315, 130 ppb in the case of HTL319,and 280 ppb in the case of HTLMix (Figure 7). The mostefficient mobilisation was in the case of HTL315; in thissample, Co was leached at a concentration of up to1070 ppb. Tungsten was also detected in every leachatethough at lower concentrations.

Ag

Co

Ga

Mo

Sb

Sr

V

W

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL315 + 0.1 M acetic acid HTL319 + 0.1 M acetic acid

Ag

Co

Ga

Mo

Sb

Sr

V

W


110

100

10000

1000

110

100

10000

1000

110

100

10000

1000

Figure 7: Concentration (in ppb) of the “at risk” elements in each sample reacted with 0.1M acetic acid at 70°C temperature under 1 barpressure in batch rotating shakers (absence of data points reflects concentrations below the limits of detection).

HTLMix + 0.1 M acetic acid 150° C

Al BBa

Ca

Cd

Cr

CuFe

KMgMnNa

Ni

Pb

Rb

SSi

Zn

0.0

1.00.1

10.0100.01000.0

(a)

Ag

Co

Ga

Mo

Sb

Sr

V

W

110

1001000

10000

HTLMix + 0.1 M acetic acid 150° C

(b)

Figure 8: Concentration of the “common” ((a) in ppm) and “at risk” ((b) in ppb) elements obtained from HTLMix at 150°C temperatureunder 200 bar pressure in Ti batch vessel after 1000 hours (absence of data points reflects concentrations below the limits of detection).

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn

HTLMix + 0.013 M mineral acid

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


0.00.1

10.01000.0

0.00.1

10.01000.0

0.00.1

10.01000.0

Figure 9: Elemental composition (in ppm) of each leachate reacted with the mixture of 0.01MHCl and 0.003MHNO3 at 100°C temperature

under 200 bar pressure in Ti batch reactors (absence of data points reflects concentrations below the limits of detection).

10 Geofluids

HTLMix was also leached using 0.1M acetic acid at150°C temperature under 200 bar pressure. Figure 8 showsthe concentrations of the selected “common” and “at risk”elements. The total concentrations of “common” and “atrisk” elements mobilised were 300 ppm and 830ppb, respec-tively. Ca, Si, and Pb had the highest abundance amongst the“common” elements, constituting 37%, 25%, and 6% of the

total “common” elements leached, respectively. Sb, Sr, andCo were the most important “at risk” elements leached, con-stituting 79%, 17%, and 3% of the total, respectively.

4.1.3. Leaching Tests Using Mineral Acid as Solvent. Leachingin Ti batch reactors using the mixture of 0.01M hydrochloricacid and 0.003M nitric acid was conducted at 100°C

Ag

Co

Ga

Mo

Sb

Sr

V

W

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL321 + 0.013 M mineral acid HTL321 + 0.013 M mineral acid

Ag

01

10100

100001000

01

10100

100001000 Co

Ga

Mo

Sb

Sr

V

W

HTL321 + 0.013 M mineral acid

110

100

10000

1000

Figure 10: Concentration (in ppb) of the “at risk” elements in each sample reacted with the mixture of 0.01M HCl and 0.003M HNO3 at100°C temperature under 200 bar pressure in Ti batch reactors after 770 hours (absence of data points reflects concentrations below thelimits of detection).

AlB

BaCa

Cd

Cr

CuFe

KMg

MnNa

Ni

Pb

Rb

SSi

ZnAl

BBa

Ca

Cd

Cr

CuFe

KMg

MnNa

Ni

Pb

Rb

SSi

Zn

HTLMix 200 °CHTLMix 100 °C

010

1000100000

01

100100001000000

Figure 11: Elemental composition (in ppm) of HTLMix reacted with the mixture of 0.1M HCl and 0.03M HNO3 at 100°C and 200°C

temperature under 200 bar pressure in Ti batch reactors after 770 hours and 530 hours, respectively.

HTLMix 200 °CHTLMix 100 °C

Ag

Co

Ga

Mo

Sb

Sr

V

W

Ag

Co

Ga

Mo

Sb

Sr

V

W

110

1001000

10000

01

10100

100010000

Figure 12: Concentration (in ppb) of the “at risk” elements in HTLMix reacted with the mixture of 0.1MHCl and 0.03MHNO3 at 100°C and200°C temperature under 200 bar pressure in Ti batch reactors after 770 hours and 530 hours, respectively (absence of data points reflectsconcentrations below the limits of detection).

11Geofluids

Table6:Physicalp

ropertiesof

each

fluid-rock

interactionin

theflow

-throu

ghreactor.

LeachateID

Rock

sampleID

Rocksampleorigin

Rock

sampletype

Rocksamplecharacteristics

Solvent

Pressure

minim

um(bar)

Pressure

maxim

um(bar)

Residence

time(m

in)

HTL3

22+DIwater

1

HTL3

22HURud

abánya

MVT

Bandedbariticlead

orefrom

ametasom

aticdepo

sitho

sted

bylim

estone

Deion

ised

water

225

290

36

HTL3

22+DIwater

2250

257

48

HTL3

22+DIwater

3250

285

35

HTL3

22+0.1M

aceticacid

0.1M

aceticacid

250

260

48

HTL3

24+DI_water

HTL3

24HURecsk

Porph

yry

Intrusion-relatedpo

rphyry

copp

erdepo

sit

Deion

ised

water

220

240

25

HTL3

24+0.1M

aceticacid

0.1M

aceticacid

250

250

38

HTL3

21+DI_water

HTL3

21RO

CacovaIerii

Skarn

Magnetitedepo

sitenriched

insulphideswithvisiblechalcopyrite

Deion

ised

water

240

248

52

HTL3

21+0.1M

aceticacid

0.1M

aceticacid

250

270

27

HTL3

15+0.1M

aceticacid

HTL3

15UKSouthCaradon

Granite-hosted

mineralisation

Mainstage

mineralisationassociated

withgranitebodies

0.1M

aceticacid

250

281

37

HTL3

19+0.1M

aceticacid

HTL3

19UKCligga

Head

Granite-hosted

mineralisation

Tin-tun

gstenmineralisation

associated

withgranitebodies

0.1M

aceticacid

218

265

42

HTLM

ix+0.1M

acetic

acid

HTLM

ixUKHerod

sfoot

Baked

sediments

withmineralised

veining

Mixture

ofvariou

ssamples

containing

galena,sph

alerite,and

somechalcopyrite

0.1M

aceticacid

250

289

36

12 Geofluids

temperature under 200 bar pressure. The total concentrationof “common” elements leached in the final sample, shown inFigure 9, was approximately 130 ppm in the case of HTLMix,150 ppm in the case of HTL321, and 250 ppm in the case ofHTL324. In the HTLMix leachate, Ca and Mg were detectedin the highest concentration, constituting 63% and 25% ofthe total “common” elements, respectively. In this sample,Mn, Na, and S were also found at notable concentrations.In the leachate reacted with HTL321, Ca and Mg were alsothe most abundant elements, constituting 63% and 26% ofthe total “common” elements leached, respectively. K, Mn,Ma, and S were also detected in lower but potentially recov-erable concentrations in this sample. In the leachate reactedwith HTL324, Fe was found in the highest concentration,constituting 58% of the total “common” elements leached.Al, Ca, Mg, and Zn were also found in concentrations of27 ppm, 32 ppm, 23 ppm, and 2ppm, respectively.

Reaction with the mixture of 0.01M hydrochloric acidand 0.003M nitric acid at 100°C under 200 bar pressureresulted in leachate concentrations of approximately125 ppb of the selected “at risk” elements in the case ofHTLMix, 160 ppb in the case of HTL321, and 65 ppb in thecase of HTL324 (Figure 10). In these samples, Co, Mo, Sb,and Sr were detected, of which Sb had the highest concentra-tions in all three samples.

HTLMix was also leached using a higher concentration(0.1M hydrochloric acid and 0.03M nitric acid), at 100°Cand 200°C, under 200 bar pressure. Figure 11 shows theconcentrations of the “common” elements in each leachate.The total concentration of “common” elements leachedwas approximately 4480 ppm at 100°C (770 hours) and213,000 ppm at 200°C (530 hours). In the case of reactionat 100°C, the most abundant element leached was Pb,which was found at a concentration of 3680 ppm in theleachate. At 200°C, Si, Fe, and Pb were the most abundantelements at 211000 ppm, 850 ppm, and 805 ppm concen-tration, respectively.

Reaction with the 0.13M mineral acid solution at 100°Cand 200°C temperature under 200 bar pressure resulted in

concentrations of approximately 340 ppb and 8500 ppb ofthe selected “at risk” elements in the final samples taken,respectively. Figure 12 shows the measured Ag, Co, Ga, Mo,Sb, Sr, V, and W concentrations. In the case of reaction at100°C, Co, Sr, and Ga were mobilised at concentrations of160 ppb, 150 ppb, and 10 ppb, respectively. Reaction at200°C resulted in the mobilisation of 7100 ppb Sb, 980 ppbCo, 220 ppb Sr, 180 ppb Mo, 8 ppbGa, and 8ppbW.

4.2. Flow-through Measurements. Experiments using theflow-through reactor described in the methods section wereconducted using deionised (DI) water and 0.1M acetic acidon samples HTL315, HTL139, HTL321, HTL322, HTL324,and HTLMix at 200 and 250°C temperature under 250°barpressure. A summary of the actual physical properties duringthe flow-through tests is represented by Table 6. Leachatesamples from each reaction were analysed using ICP-MS.Data about the composition of samples from the flow-through reaction, on which the following figures are based,can be found in Tables 7 and 8.

4.2.1. Leaching Tests Using Deionised Water. Flow-throughleaching experiments were carried out using deionised wateron samples HTL321, HTL322, and HTL324 at 200°C. Theconcentration of “common” elements leached can be seenin Figure 13, and the concentration of “at risk” elementsmobilised from solid samples is presented in Figure 14.

The total concentration of the selected “common” ele-ments in HTL321 is approximately 1000 ppm (of which380 ppm is Ca and 380 ppm is S); for HTL322, the total lea-ched is 90 ppm (of which 40 ppm is Ca and 27 ppm is S);for HTL324, the total is 940 ppm (of which 480 ppm is Caand 360ppm is S). Reaction with deionised water resultedin a total concentration of 400 ppb of the selected “at risk”elements in the case of HTL321, 700 ppb in the case ofHTL322, and 520 ppb in the case of HTL324. The highestconcentration element among the “at risk” elements was Sr,representing 94%, 94%, and 76% of the total amount of “at

Table 7: Concentration of the “at risk” elements in each leachate from the flow-through reaction.

Element Ag Co Ga Mo Sb Sr V W Total ‘at risk’Sample PPB PPB PPB PPB PPB PPB PPB PPB PPB

HTL322 +DI water 1 ∗ 4.24 ∗ 0.6 19.45 844.57 ∗ 0.19 869.05

HTL322 +DI water 2 ∗ 2.77 ∗ 0.6 15.81 660.15 ∗ 0.07 679.40

HTL322 +DI water 3 ∗ 2.02 ∗ 0.5 65.22 489.79 ∗ 0.05 557.58

HTL322 + 0.1M acetic acid ∗ 0.81 ∗ 8.6 91.66 414.46 ∗ 0.23 515.76

HTL324 +DI_water 0.09 0.28 ∗ 3.8 18.75 377.31 0.3 1.06 401.59

HTL324 + 0.1M acetic acid ∗ ∗ ∗ ∗ ∗ 2837.00 ∗ ∗ 2837.00

HTL321 +DI_water ∗ ∗ ∗ ∗ ∗ 1526.00 ∗ ∗ 1526.00

HTL321 + 0.1M acetic acid ∗ ∗ ∗ ∗ ∗ 1094.00 ∗ ∗ 1094.00

HTL315 + 0.1M acetic acid ∗ 209.29 ∗ ∗ ∗ 8.38 10 ∗ 227.67

HTL319 + 0.1M acetic acid 90 9.2927 ∗ ∗ 440 95.15 ∗ 470 1104.45

HTLMix + 0.1M acetic acid 20 94.967 ∗ ∗ ∗ 918.33 ∗ ∗ 1033.30∗: concentration was under the detection limit.

13Geofluids

Table8:Con

centration

ofthe“com

mon

”elem

entsin

each

leachatefrom

theflow

-throu

ghreaction

.

Element

Al

BBa

Ca

Cd

Cr

Cu

FeK

Mg

Mn

Na

Ni

Pb

Rb

SSi

Zn

Sample

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

PPB

HTL3

22+DIwater

121

62248

52,890

140

23

∗2940

4290

577

1530

421314

936,000

4576

5895

HTL3

22+DIwater

24

45228

47,650

117

22

∗3430

3420

420

1830

251067

1131,000

4237

5041

HTL3

22+DIwater

312

27220

21,240

791

16∗

1390

1580

266

1690

717,008

414,000

1740

5768

HTL3

22+0.1M

aceticacid

4289

107

477,120

∗1

9∗

10,820

26,190

960

8490

652

81364,000

51,658

27

HTL3

24+DI_water

767105

155

383,830

∗8

8∗

8180

126,020

286

18,800

319

60379,000

74,144

16

HTL3

24+0.1M

aceticacid

∗479

1958

378,200

2577

∗∗

∗54,000

44,000

645

10,000

∗537,892

∗22,000

∗98,738

HTL3

21+DI_water

∗439

268

1,261,900

∗∗

∗90,925

41,000

24,000

8896

63,000

∗5299

∗182,000

∗1702

HTL3

21+0.1M

aceticacid

∗12,906

153

1,314,300

∗∗

∗29,330

7000

375,000

1904

17,000

∗22,190

∗375,000

∗934

HTL3

15+0.1M

aceticacid

2548

1718

2636,719

∗145

4512,812

21,313

11,997

∗6186

356

1145

∗∗

∗6292

HTL3

19+0.1M

aceticacid

645

399

717793

∗201

1354,192

7450

2344

∗4169

290

98∗

∗∗

208

HTLM

ix+0.1M

aceticacid

98678

639

609,030

8413

14805

49,918

117,900

∗29,278

154

243,620

∗∗

∗7969

∗:con

centration

was


it.

14 Geofluids

risk” elements in the leachates, respectively. Sb and W werealso detected amongst other potentially useful elements.

4.2.2. Leaching Tests Using Acetic Acid as Solvent. Flow-through leaching experiments were also carried out using0.1M acetic acid. Total dissolved solids (TDS) concentrationsin the leachates from these experiments were too high to usethe ICP-MS analysis previously employed; therefore, aninstrument meant for midgrade concentrations, ICP-OES,was used. This method had a higher detection limits for somecrucial elements; therefore, it was useful only in the determi-nation of the concentrations of the most abundant elements.The concentrations of elements in the leachate as a result ofthe reaction can be seen in Figure 15.

The total concentrations of “common” elements leached,shown in Figure 15, correspond to approximately 1060 ppmin the case of HTLMix, 100 ppm in the case of HTL315,80 ppm in the case of HTL319, 2150 ppm in the case ofHTL321, 1150 ppm in the case of HTL322, and 1680ppmin the case of HTL324. Generally, the most abundant elementin the leachates was Ca, constituting 57%, 36%, 10%, 61%,33%, and 75% of the total “common” elements in

leachates from experiments using HTLMix, HTL315,HTL319, HTL321, HTL322, and HTL324, respectively. Inthe case of HTLMix and HTL322, enhanced Pb release wasexperienced with concentrations of 240 ppm and 540ppmin the leachates from these samples, respectively.

Reaction with 0.1M acetic acid at 250°C temperatureunder 250 bar pressure resulted in total concentrations ofthe selected “at risk” elements of 1030 ppb, 230 ppb,1100 ppb, 1100 ppb, 2840 ppb, and 1530 ppb for leachatesfrom experiments using HTLMix, HTL315, HTL 319,HTL321, HTL322, and HTL324, respectively. Figure 16shows the concentrations of Ag, Co, Ga, Mo, Sb, Sr, V, andW in these samples.

In HTLMix, Ag was detected at a concentration of20 ppb. In HTL315, 10 ppb of V was detected together withSr and Co. The widest range of elements was mobilised fromHTL319 with leachate concentrations of 440 ppb Sb, 95 ppbSr, 90 ppb Ag, and 9ppb Co;W was also detected in a notableamount at a concentration of 470 ppb. In the cases ofHTL321, HTL322, and HTL324, only Sr was detected inone of the largest concentrations in the leachates at concen-trations of 1090 ppb, 2840 ppb, and 1530 ppb, respectively.

AlB

BaCa

Cd

Cr

CuFe

KMg

MnNa

Ni

Pb

Rb

SSi

Zn

HTL322 + DI water

AlB

BaCa

Cd

Cr

CuFe

KMg

MnNa

Ni

Pb

Rb

SSi

Zn

HTL324 + DI water

AlB

BaCa

Cd

Cr

CuFe

KMg

MnNa

Ni

Pb

Rb

SSi

Zn

HTL321 + DI water

0.00.1

10.01000.0

0.00.1

10.01000.0

0.00.1

10.01000.0

Figure 13: Elemental composition (in ppm) of each leachate from samples reacted with deionised water at 200°C temperature under 250 barpressure in the flow-through reactor after 52 minutes, 48 minutes, and 25 minutes, respectively (absence of data points reflects concentrationsbelow the limits of detection).

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL322 + DI water

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL324 + DI water

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL321 + DI water

0

1

10010000

0

1

10010000

0

1

10010000

Figure 14: The amount of “at risk” elements in leachate from samples reacted with deionised water at 200°C temperature under 250 barpressure in ppb after 52 minutes, 48 minutes, and 25 minutes, respectively (absence of data points reflects concentrations below the limitsof detection).

15Geofluids

5. Discussion

5.1. Comparison of Different Fluids. The tests in this studywere designed largely to test the efficacy of different leachingagents. While aggressive fluids, such as mineral acids, clearlyhave more potential to dissolve rock and associated minera-lisation, this is not the only consideration in such a system.Also, of primary concern are the environmental impact ofthe fluids used and the impact on the reservoir and infra-structure. While a mineral acid may leach larger concentra-tions of desirable elements, it is also likely to attack bulkrock, leaching less desirable elements. The latter may requireconsideration when it comes to reinjection or disposal ormay cause problems of precipitation in the system (as com-monly occurs for silica). Likewise, such fluids are likely toattack borehole and surface infrastructure materials andhave a more environmental impact. More benign fluids,such as fresh water, mild acids, or organic leachants, aremore likely to be publicly and environmentally acceptablebut have the obvious trade-off of weaker leaching effectsand potential instability at elevated temperatures (particu-larly in the case of organic leachants). Some investigationof the relative performance of potential leachants is thereforerequired and a comparison between our chosen leachants isprovided below.

For each fluid studied (deionised water, 0.1M acetic acid,and with 0.13M mineral acid), one experiment was chosen

from the batch tests to compare the dissolution propertiesof each fluid. Experimental runs which were conductedunder very similar conditions were selected, and results fromthese are shown in Figure 17 for “common” elements and inFigure 18 for “at risk” elements. The figures include datafrom HTLMix reacted with deionised water at 200°C under200 bar pressure for 670 hours with 0.1M acetic acid at150°C under 200 bar pressure for 1000 hours and with0.13M mineral acid at 200°C under 200 bar pressure for530 hours.

The sum of mobilised “common” elements is approxi-mately 80 ppm, 300 ppm, and 213,000 ppm in the case ofdeionised water, 0.1M acetic acid, and 0.13M mineral acid,respectively. The huge increase in total dissolved elementsis largely due to the previously mentioned large amount ofSi mobilised (approximately 211,000 ppm) in the case of themineral acid experiment. This considerable attack on the sil-icates (largely quartz) present in the system by the moreacidic fluids and such extremely high Si concentration indi-cates that the solution was highly effective at dissolving thematrix minerals. Such dissolution, leading to a reduction insolids volume and the associated increase in porosity, couldexpose more surfaces of metal-bearing minerals and alsoincrease permeability and enhance fluid flow through thesystem. However, a higher dissolved load could also causemore precipitation on cooling. This highlights the dualityof such fluids, enhancing reservoirs through increased

AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


AlB

Ba

Ca

Cd

Cr

Cu

FeK

MgMn

Na

Ni

Pb

Rb

S

SiZn


0.11

10100

100010000

0.11

10100

100010000

0.11

10100

100010000

0.11

10100

100010000

0.11

10100

100010000

0.11

10100

100010000

Figure 15: Elemental composition of each leachate reacted with 0.1M acetic acid at 250°C temperature under 250 bar pressure in the flow-through reactor after 36, 37, 42, 27, 38, and 48 minutes, respectively (absence of data points reflects concentrations below the limits ofdetection).

16 Geofluids

permeability but potentially also leading to issues withprecipitation in surface infrastructure.

Total concentrations of “at risk elements” are approxi-mately 50 ppb in the case of reaction with deionised water,830 ppb in the case of reaction with acetic acid, and8500 ppb when mineral acid was used. There is a strong rela-tion between acidity and the total amount of mobilised ele-ments. In our study, during batch experiments at highertemperatures, mineral acid was the best solvent, resulting inrelatively high metal mobilisation and high concentrationof Al and Si, which in practice would mean a higher riskfor scaling. Leaching with acetic acid resulted in moderatemetal concentration and significantly lower Al and Si con-centrations, suggesting this as a viable leaching option anda good compromise between useable metal release and scal-ing risk. Also, it could be more challenging to use mineralacids as leaching solvent in geothermal systems due to diffi-culties around transportation, handling, storage, and envi-ronmental issues. In terms of mobilised “at risk” elements,acetic acid was more effective than deionised water but lessso than mineral acid (both in terms of dissolved concentra-tions and the spread of elements leached in detectableconcentrations). Even though it is difficult to quantify theenvironmental impact, the additional leaching potential ofmineral acid is unlikely to make use of such an aggressivefluid, even when used at relatively dilute concentrations ashere, acceptable. The results indicate, however, that even arelatively mild leachant, such as acetic acid, with which the

public is familiar as an everyday substance, can substantiallyincrease leaching potential. In this case, a switch from deio-nised water to acetic acid generates a nearly 20-foldincrease in the dissolved load of the selected “at risk” ele-ments. Naturally, acetic acid may not be suitable for useunder all circumstances; organic acids have a tendency tobreak down at elevated temperatures for example (thoughthe natural breakdown of an injectant may be seen as desir-able in the long term), but these results highlight the potentialof using such, relatively simple, fluids to enhance leaching inan EGS system.

5.1.1. Batch Experiments. A time series of elemental con-centrations from HTLMix reacted with 0.13M mineral acidat 100°C temperature under 200 bar pressure in a batchreactor is shown in Figure 19. Samples shown in the figurewere collected after 70 hours, 290 hours, and 770 hoursof reaction.

The total concentration of the selected elements in thefirst sample was approximately 4190 ppm, 4220 ppm in thesecond sample, and 4480 ppm in the third sample. The trendshows a slight, but steady, increase in leachate concentrationsover time for most elements. The notable exception to thistrend is iron, which after an initial increase can be seen todecrease over the course of the experiment, indicating somereprecipitation or scavenging of this element. Leachingefficiency is largest at the beginning of the experiment anddecreases over time in the case of every element.

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL MIX + 0.1 M acetic acid

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL 315 + 0.1 M acetic acid

Ag

Co

Ga

Mo

Sb

Sr

V

W

HTL 319 + 0.1 M acetic acid

Ag

Co

Ga

Mo

Sb

Sr

V

W


Ag

Co

Ga

Mo

Sb

Sr

V

W


Ag

Co

Ga

Mo

Sb

Sr

V

W


0

1100

10000

0

1100

10000

0

1100

10000

0

1100

10000

0

1100

10000

0

1100

10000

Figure 16: The amount of “at risk” elements in leachate samples reacted with 0.1M acetic acid at 250°C temperature under 250 bar pressure inppb after 36, 37, 42, 27, 38, and 48 minutes, respectively (absence of data points reflects concentrations below the limits of detection).

17Geofluids

Exceptionally high concentrations of Pb were detected atconcentrations of approximately 3530 ppm, 3520 ppm,and 3680 ppm after 70 h, 290 h, and 770 h reaction, respec-tively. Leaching was most rapid in the first 70 hours,mobilising 3530 ppm Pb, and then decreased radically,only increasing Pb concentrations by 150 ppm over theremaining 700 hours of the experiment, as the systemapproached equilibrium.

Available surface area for a chemical reaction (and heatexchange) is crucial for raw material mobilisation, as miner-alised rock formations dissolve quickly, suggesting thatincreasing available surface through stimulation might bemore effective in increasing dissolved load rather thanincreasing the path length or residence time in the reservoir[19]. On a reservoir scale, this would mean that the targetedmetal-rich ore formation is depletable and extraction wouldbe most efficient early on in an extraction project withreturns likely to decrease relatively rapidly over time [20].The lifespan of such a metal extraction facility would behighly dependent on the size and grade of the geological for-mation but would likely be shorter than the plant life of a

geothermal energy-producing facility without managing thereservoir in such a way that injected fluid could contact freshsurfaces regularly, i.e., through redirection of groundwaterflow or through mechanical stimulation of the reservoir,creating fresh surfaces.

5.1.2. Flow-through Measurements. In addition to fresh sur-face area, another limiting factor in potential metal extractionwill be the volume of “fresh” fluid that can be passed throughthe reservoir. In the batch experiments above, reactions canbe seen slowing dramatically within a few 10s of hours asequilibrium is approached. Therefore, comparison withflow-through experiments can be instructive. A time seriesof elemental concentrations from one of the flow-throughexperiments is shown in Figure 20. Samples shown in the fig-ure represent concentrations from reactions in the first 36minutes, 37–84 minutes, and 85–119 minutes.

The total concentration of the shown elements in the firstsample was approximately 110 ppm, 98 ppm in the secondsample, and 65 ppm in the third sample. This trend in leach-ate concentrations is not element specific; the concentrations

1.00E + 00

1.00E + 01

1.00E + 02

1.00E + 03

1.00E + 04

Ag Co Ga Mo Sb Sr V W Total

Comparing the effect of different fluids in the case of “at risk” elements

Deionised water, 200°C, 200 bar0.1 M acetic acid, 150°C, 200 bar0.13 M mineral acid, 200°C, 200 bar

Conc

entr

atio

n (p

pb)

Figure 18: Comparison of the effectiveness of different fluids for “at risk” elements with concentrations in ppb.

1.0E − 03

1.0E − 02

1.0E − 01

1.0E + 00

1.0E + 01

1.0E + 02

1.0E + 03

1.0E + 04

1.0E + 05

1.0E + 06

Al B Ba Ca Cd Cr Cu Fe K Mg Mn Na Ni Pb Rb S Si Zn Total

Conc

entr

atio

n (p

pm)

Deionised water, 200°C, 200 bar0.1 M acetic acid, 150°C, 200 bar0.13 M mineral acid, 200°C, 200 bar

Comparing the effect of different fluids in the case of “common” elements

Figure 17: Comparison of the effectiveness of different fluids for “common” elements with concentrations in ppm.

18 Geofluids

of Ca (53 ppm, 47 ppm, and 21 ppm), Mg (4 ppm, 3 ppm, and1.5 ppm), Mn (580 ppb, 420 ppb, and 270 ppb), S (35 ppm,30 ppm, and 15 ppm), and Si (5 ppm, 4 ppm, and 1.5 ppm)amongst others show this trend, too. Even though the con-centration of total dissolved “common” elements increasedover time, the amount of increase decreased.

The tendency shown by these graphs indicates a rapidleaching reaction (for the first sample the contact time was

36 minutes). Dissolution of solid material occurs most rap-idly during earlier times and then continues at lower dissolu-tion rates. This is a key factor to consider when designing thecirculation rate of an EGS system, which controls the resi-dence time of the fluid in the reservoir. When combiningmetal leaching with the harnessing of geothermal energy,the rock dissolution processes for metal mobilisation wouldhave longer contact times than those demonstrated here;

1

10

100

1000

10000


Conc

entr

atio

n (p

pm)

“Common” element concentration time series

70 h290 h770 h

Figure 19: The kinetics of elemental composition over time in leachates from HTLMix sample reacted with 0.13M mineral acid at 100°Ctemperature under 200 bar pressure in the batch reactor after 70, 290 and 770 hours, respectively.

1.0E − 03

1.0E − 02

1.0E − 01

1.,0E + 00

1.0E + 01

1.0E + 02

1.0E + 03


�e amount of each “common” element in HTL322 over time

0-36 min37-84 min85-119 min

Conc

entr

atio

n (p

pm)

Figure 20: The kinetics of elemental composition over time in leachates from HTL322 sample reacted with deionised water at 200°Ctemperature under 250 bar pressure in the flow-through reactor after 36, 84, and 119 minutes, respectively.

19Geofluids

therefore, results from this study could be considered as min-imum thresholds.

In both experimental setups, the solid sample used waspowdered rather than a whole rock. This form of materialmight have several high energy sites caused by the samplepreparation, leading to relatively high dissolution rates overthe course of the experiments. This might also be the casein a geothermal reservoir which has been stimulated. Newfresh surfaces with which the fluid may react, leading to highinitial loads, which will tail off with time. If no stimulation isbeing carried out, dissolution may be lower, but steady. Thefresh surface area in a reservoir will be relatively low, butthe contact time will be much longer than that shown here.Contact time for fluid-rock interactions in case of batch reac-tions was average 30 days maximum, while flow-throughtests had less than 1 hour contact time (but much more solidmaterial and hence available surface area). In an operatingEGS plant in Europe, the average travel time of the fluidwas 23 days, depending on the flow path [21, 22]. It will beimportant to consider these effects when looking at the wholelife-cycle of a project, possibly resulting in high recovery

initially, tailing off later on, without continued stimulationor exposure of new surfaces.

5.1.3. Summary. While batch experiments are useful in rap-idly elucidating relative leaching potentials of various fluids,the results they produce are often difficult to compare to“real” systems due to very high fluid-rock ratios and the factthat they represent a closed system with no input of freshfluid. As part of this work, therefore, flow-through experi-ments were also carried out, allowing a direct comparisonbetween the two system types. Figure 21 is a summary ofthe experiments conducted with deionised water and aceticacid. It displays the total concentrations of “common” and“at risk” elements from all initial solid samples and next toit in each leachate. Data about the composition of solid rocksamples, on which the following figures are based, can befound in Tables 9 and 10.

The data in Figure 21 has been broken down to Figure 22,which compares the total concentration of “common” ele-ments in the final leachates, and Figure 23, which focuseson the more critical “at risk” elements.

"com

mon

""a

t risk

""c

omm

on"

"at r

isk"

"com

mon

""a

t risk

""c

omm

on"

"at r

isk"

"com

mon

""a

t risk

""c

omm

on"

"at r

isk"

"com

mon

""a

t risk

""c

omm

on"

"at r

isk"

"com

mon

""a

t risk

""c

omm

on"

"at r

isk"

"com

mon

""a

t risk

""c

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on"

"at r

isk"

"com

mon

""a

t risk

""c

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on"

"at r

isk"

"com

mon

""a

t risk

"

"com

mon

""a

t risk

""c

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on"

"at r

isk"

"com

mon

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"

"com

mon

""a

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""c

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on"

"at r

isk"

"com

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""a

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"

HTLMix HTL315 HTL319 HTL321 HTL322 HTL324

1.0E + 00

1.0E + 01

1.0E + 02

1.0E + 03

1.0E + 04

1.0E + 05

1.0E + 06

1.0E + 07

1.0E + 08Comparison of the effectiveness of each leaching method

Original content of the solid Batch deionised water, 70°C, 670 hBatch 0.1M acetic acid, 70°C, 720 h Flow-through deionised water, 200°C, 0.6 hFlow-through acetic acid, 250°C, 0.6 h

Figure 21: Comparison of rock sample composition to results from the batch and flow-through leaching experiments with deionised waterand acetic acid as solvent. Important to note that the axis is logarithmic and the concentrations of “common” elements are in ppm and theconcentrations of “at risk” elements are in ppb.

Table 9: Concentration of the “at risk” elements in each solid rock sample.

Element Ag Co Ga Mo Sb Sr V W Total ‘at risk’Sample PPB PPB PPB PPB PPB PPB PPB PPB PPB

HTLMix 45258.83 4169.87 4272.66 223.06 861478.98 16679.67 20292.37 42936.29 995311.73

HTL315 11973.44 249443.41 9746.84 780.91 145129.71 15440.17 7738.55 216446.11 656699.15

HTL319 639.66 430.16 1195.83 1533.93 5513.29 9968.94 718.91 17678896.30 17698897.02

HTL321 424.80 78783.79 1392.45 700.33 58165.19 15421.11 22437.25 25124.42 202449.34

HTL322 534398.93 4028.59 1282.41 2386.12 2497731.37 1156992.59 2913.91 9290.47 4209024.39

HTL324 3098.11 48344.41 17976.82 20344.31 694.81 5236.13 258590.19 6466.08 360750.86

20 Geofluids

Table10:C

oncentration

ofthe“com

mon

”elem

entsin

each

solid

rock

sample.

Element

Al

BBa

Ca

Cd

Cr

Cu

FeK

Mg

Mn

Na

Ni

Pb

Rb

SSi

Zn

Sample

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

PPM

HTLM

ix15530.48

∗46

4205

3725

641

5993

5880

2974

350

551

983,238

4014,437

408,010

6814

HTL3

1516406.93

∗18

374

124

13,412

132,105

868

1927

765

510

828

891,747

330,518

249

HTL3

193438.966

∗11

109

028

7416,407

1262

514656

104

127

2659

412,250

68

HTL3

21517.3251

∗8

51,541

010

1445

397,790

3437,169

782

558

70

217,122

3290

142

HTL3

221292.076

∗2503

9029

1075

148

16,480

546

2013

301

627

59,105

22729

39,010

178,876

HTL3

2411615.29

∗16

1485

15

11,585

186,533

1766

4374

561

851

3312

875,251

345,450

189

∗:con

centration

was


it.

21Geofluids

As shown in Figure 22, acetic acid has a much betterpotential to dissolve “common” elements relative to deio-nised water. The experiment carried out using HTL322 anddifferent fluids is a good example for this; acetic acid (mobi-lising 0.368% of the original content) resulted in an almost 13times increase in the concentration of “common” elementsover deionised water (mobilising 0.029% of the original con-tent). Based on these samples, however, flow-through reac-tions yielded in slightly higher total dissolved elementconcentrations than batch reactions, even over relativelyshort contact times. This in part was due to a considerablylower fluid to rock ratio used in the flow-through experi-ments, which also indicates that the availability of fresh sur-face area is an important factor in determining leachingrates. Given the rapidity of reaction in the flow-throughexperiments, these results indicate that even in a reservoirwhere fresh surface area is more limited than that used here,reasonably high dissolved loads should be achievable, espe-cially given the much greater contact times between fluidand mineral in such systems. The spider plots of these reac-tions show similar patterns, in terms of leachate elementalcompositions, in both experimental setups.

In some of the flow-through experiments (e.g., flow-through reaction with 0.1M acetic acid and HTL321), ele-ments are observed in the leachates at very elevated(>1000ppm) concentrations. Generally, Ca, Fe, Mg, and Swere the most abundant elements in the leachates, all of

which are associated with scale formation. During laboratorytests, no scaling in the equipment or in the tubing wasobserved. Figure 23 shows only the concentration elementswhich we have defined as “at risk”.

The trend of mobilisation of “at risk” elements correlatesto that of “common” elements. Acetic acid proved to be farmore effective than deionised water in all cases. One of themost effective dissolution of “at risk” elements was in the caseof HTL321 reacted with 0.1M acetic acid, where 0.54% of theoriginal content could be mobilised. This is 1.5 times moreeffective than the best result at “common” elements. Sr, Co,W, and Mo elements were leached at the highest concentra-tions, indicating the potential for further investigations dueto the criticality of tungsten and cobalt.

6. Conclusions

Laboratory batch and flow-through leaching tests were run inthe range of 70–250°C under 1–300 bar pressure, conditionswhich include the properties of an average geothermal reser-voir at 3 km depth. As a result of the reactions in the labora-tory, reasonable concentrations of a wide range of elementscould bemobilised. Detected elements were grouped as “com-mon” and “at risk” elements. Selected “common” elementshave less economic importance and higher occurrence (inboth the solid samples used and the produced leachates),

1 10 100 1000 10000 100000 1000000

Original content of the solidBatch deionised waterBatch 0.1M acetic acid

Flow-through acetic acid





Original content of the solidFlow-through deionised water






HTL

Mix

HTL

315

HTL

319

HTL

321

HTL

322

HTL

324

�e total amount of “common” elements in each leachate

70°C, 1 bar, 670 h70°C, 1 bar, 720 h

200°C, 250 bar, 0.6 h250°C, 250 bar, 0.6 h

Figure 22: Summarising chart of detected “common” elements during each experiment with concentrations in ppm.

22 Geofluids

while our selected “at risk” elements have higher economicvalue, and lower supply security.

One of the highest concentrations of “common” elementsoccurred in the case of lead, which mobilised at concentra-tions of up to 870 ppm with acetic acid during the batch reac-tion and up to 540 ppm in the flow-through reactor. Notableconcentrations of Zn are also present in leachates, whichcorroborates the enhanced dissolution progress. In labora-tory tests, significant Al and Si were also found. Elevatedconcentrations of these elements indicate considerable disso-lution of matrix silicates present in the samples. This could bedesirable in terms of increasing reservoir permeability andopening flow paths, but if concentrations become too high,there is an increased risk of precipitation, which could clogfractures and inhibit fluid flow in a geothermal reservoir,and risk fouling boreholes or surface infrastructure. In a tech-nologically optimised geothermal reservoir, where extractionwould target metals but not necessarily in their pure forms(i.e., could include extraction of metals complexed with someof the scale-forming elements), therefore as they are removedfrom the fluid in a technological material extraction step, therisk of clogging would decrease in the reservoir and in thewell. In this way, metal extraction would reduce natural scal-ing and therefore increase efficiency.

The highest concentration of a single element detectedfrom the “at risk” group was 1070 ppb Co concentration inbatch reactors with acetic acid and 2840 ppb Sr concentrationin a flow-through setup with acetic acid. During all leaching

tests, Sr, Co, W, and Mo were detected with the largest abun-dance, which is a good motivation towards further experi-ments as tungsten and cobalt have the highest economicrisk rating for the EU, respectively.

In leaching reactions, even reasonably mild and environ-mentally acceptable fluids could be utilised to dissolve consid-erable amounts of silicate material as well as some elements ofinterest. Fast reaction rates given the right conditions (hightemperature and pressure with good amount of available sur-face area) within the geothermal reservoir are promising interms of potential to metal recovery. Future work and tech-nological development is still needed to practically recoverraw materials from geothermal fluids, as the desired concen-trations for extraction tend to be higher than those achievedin this study. The results are intended to use for upscaling toreservoir scale and calculate likely dissolved loads achievable,given reaction rates and solubility of the various elementsinvolved. In this study, the first steps were done to ensurethe sustainability of the proposed technology, further investi-gations, advances in other technologies, and a full life cycleassessment study need to follow.

Data Availability

The ICP-MS and ICP-OES analysis data used to support thefindings of this study are included within the article andwithin the appendices. All other previously reported (micro-scopic and XRD analyses) data used to support this study are

1 10 100 1000 10000 100000 1000000 10000000













HTL

Mix

HTL

315

HTL

319

HTL

321

HTL

322

HTL

324

�e total amount of “at risk” elements in each leachate

70°C, 1 bar, 670 h70°C, 1 bar, 720 h

200°C, 250 bar, 0.6 h250°C, 250 bar, 0.6 h

Figure 23: Summarising chart of detected “at risk” elements during each experiment with concentrations in ppb.

23Geofluids

available at http://www.chpm2030.eu. These prior studies(and datasets) are cited at relevant places within the text.

Conflicts of Interest

The authors declare that there is no conflict of interestregarding the publication of this paper.

Acknowledgments

The CHPM2030 project has received funding from theEuropean Union’s Horizon 2020 research and innovationprogramme under grant agreement No. 654100. The authorswould like to express their gratitude to Richard Shaw, JeremyRushton, Alicja Lacinska, Dániel Fűzéri, Simon Chenery,Andrew Marriot, Elliott Hamilton, Michael Watts, IanMountney, and Simon Kemp for providing support and anal-ysis for the BGS experimental program.

References

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24 Geofluids

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